This invention relates to internal mirror gas lasers, and more particularly to an improved design of such lasers providing superior optical and structural characteristics.
Internal mirror gas lasers employ an envelope forming a gas discharge tube as a resonator support structure where laser mirrors are sealably mounted directly to the ends of the tube. The internal mirror arrangement has several advantages, including simplicity in operation and maintenance.
Lasers, particularly continuous discharge argon-ion lasers, are relatively energy inefficient in operation, resulting in the generation of significant waste heat which must be dissipated in the laser body. Argon-ion lasers operating in a continuous mode may generate waste heat on the order of 50-200 watts per centimeter of bore length (as compared to 0.4 watts per centimeter of bore length for a helium-neon laser), along the positive column or main bore of the discharge tube. This can cause differential heating of the laser components, warping and structural misalignment of the laser affecting the optical characteristics of the laser output beam. Accordingly, many argon-ion lasers utilize a resonator support structure that is thermally and mechanically isolated from the discharge tube to prevent thermal and mechanical misalignment of the resonator during operation. In external mirror configurations the laser mirrors are attached to each end of the resonator support structure and are generally adjustable to bring the optical axis of the resonator into alignment with the longitudinal axis of the bore of the discharge tube. However, external mirror structures are by comparison larger, less rugged and more complex than internal mirror structures. There is therefore a need for compact, rugged and simple lasers which maintain mirror alignment in various thermal and mechanical environments.
Various techniques have been suggested to improve heat dissipation efficiency of continuous mode lasers. For example, convection cooling is common. Forced air or flowing water cooling is generally applied to the envelope to dissipate the heat generated in the discharge tube. In the past, radially extending cooling fins have been attached to the envelope containing the main bore to increase the efficiency of heat dissipation.
Argon-ion internal mirror lasers are known which employ cooling fins in general. For example, the Toshiba Review in an article by Matsuda et al. in Nov-Dec 1979 describes an Air-Cooled Argon-Ion Laser which is of internal mirror construction having a beryllia ceramic as the capillary material in the plasma tube and rings of radially disposed cooling fins mounted to the plasma tube. The cathode housing is constructed of quartz and does not employ cooling fins. The array appears to be constructed by stacking rings on the plasma tube. McMahan, U.S. Pat. No. 3,763,442 illustrates an external mirror design in which fins are constructed by fan folding a foil sheet of thermally conductive pliant material such as copper which surrounds a beryllium oxide plasma tube. The fan fold construction permits the fin structure to adjust to the differential movement of the ceramic plasma tube during thermal cycling without damage to the structure. The cathode housing is of Kovar and does not employ cooling fins. Cooling fins of such a design as taught by McMahan or Matsuda have a number of disadvantages. First, such fin structures are difficult and expensive to fabricate with precision. Second, the structure fails to provide a suitable cooling gradient to minimize the mechanical effects associated with thermal expansion and contraction of the internal mirror laser structure. There is considerable room for improvement to achieve the goal of adequate heat dissipation and minimal thermally-induced movement of the plasma tube.
Other internal mirror lasers are known, such as those manufactured by Spectra-Physics of Mountain View, Calif. The Spectra-Physics lasers employ a cathode housing constructed of alumina which does not employ cooling fins. Heretofore the problem of controlling and dissipating heat in the cathode housing of an internal mirror laser has not been completely recognized or fully addressed.